If your system has a complexity of X to build, and using a traditional RDBMS solves Y% up front, it adds an ongoing Z complexity overhead in growing/maintaining the system over time. NoSQL may have a lower Y, but it 's goal is to also have a lower Z, and can often win out in the bigger picture even without talking about massive scale. It's not true all the time, but in my experience it's been the general rule:)

In any sufficiently complex system a combination of both usually work well together. I heard a good offhand comment this week of "80% of the structure should be in an RDBMS and 80% of the data should be via NoSQL" which makes a lot of sense to me.

I love the allusions, ties to earthly terminology, familiar yet alien setting, deep history and constant geeking out. It's a tremendous book so don't listen to the reviewer.
Imagine it as an alien story that someone actually translated/localized in every way possible, then consider what math and theory as a fundamental religion might do for long term stability.

We also already have one of these in the US, a decommissioned underground facility converted to an ultra-secure datacenter with green power, http://usshc.com/ who also hosts a number of open source projects like http://jabber.org/ and has stellar service and commodity rates.